Preparation of metal surfaces for enameling



United States Patent 6 Claims. (61. 117-53 The invention relates to the preparation of metal sheets for the application of vitreous enamels. More particularly, it relates to an etching treatment with polyhydroxycarboxylic acids or their salts, respectively, yielding silverwhite metal surfaces,

It is known that the surface pretreatment of sheet iron and steel, especially of preformed sheets, greatly influences the ensuing enameling. Therefore, the sheets not only are carefully degrease'd, but other inter-mediate treatment steps are carried out, such as etching and, if required, neutralization. Acid treatments with sulfuric or hydrochloric acid necessarily leads to the formation of iron salts and also of the salts of those metals which are present in the steel sheet as deliberate or accidental alloying components. For this reason, etching is followed by a rinsing step wherein not only the acid film, but also the salts formed are to be removed. Since basic salts readily form which frequently adhere rather tenaciously to the sheet and can be removed only with difliculty especially from the pores of the sheets, a plurality of rinsing baths often is employed, and each of these baths is adjusted to a suitable pH value.

However, faults or flaws in the vitreous enamel coatings even occur when several rinse baths are employed.

' These flaws again are the result of unremoved salt residues; Therefore, it has become the practice to add a further treatment, after rinsing with an alkaline sodium cyanide solution. The remaining salt thereby is bound in complex form, and these complexes can be removed by further rinsing and passivation treatment of the metal sheets. Whereas this process largely eliminates waste products, it has the grave disadvantage that the cyanide baths must be kept under strong surveillance in order to obviate a carryover of residual acid and thus the formation of hydrocyanic acid. For the same reason, cyanidecontaining solutions cannot be used in enamel spray processes since even the influence of carbon dioxide present in the atmosphere tends to form hydrocyanic acid.

It now has been found that the disadvantages named can be eliminated by the process according to the invention. This process provides for an etching of the metal surfaces and a prior treatment with solutions containing polyhydroxy-carboxylic acids, preferably containing at least 3-hydroxyl groups in their molecules, or with acids having the generic Formulae 1 or 2, respectively.

atoms, preferably an alkylor phenyl-radical; n is a whole number from 1 to 8; and X is OH or NH Patented May 2, 1967 The corresponding water-soluble salts may be substituted totally or in part for the acids named,

After this pretreatment, the metal sheets are passivated in the conventional manner with alkaline solutions.

The hydroxyor amino-diphosp'honic acids according to Formula 1 can 'be produced either by the reaction of mon-ocarboxylic acid anhydrides and/or chlorides with phosphorous acid or else according to the process disclosed in German Patent 1,002,355. The hydrocarbon radical R may be an alkyl radical having 1 to 8 carbon atoms or an aromatic radical.

The bydroxyor amino-tetraphosphonic acids of Formula 2 can be produced in a similar manner, whereby, however, in lieu of the monocarvboxylic anhydrides or chlorides, the corresponding dicarboxylic acid derivatives are used, or in the case of aminotetraphosphonic acids, organic dinitriles, opportunely approximately 4 mols phosphorus trihalide per mol dinitrile.

Polyhydr-oxycarboxylic acids applicable to the process according to the invention are compounds having th formulae CH OH (CHOH 3 -COOH CH OH(CHOH) COOH, COOH(CHOH) 3 COOH and COOH(CHOHMCOOH Particularly readily available and especially suited for the process are gluconic acid, saccharic acid and glucosacc'haric acid.

In lieu of the acids, their corresponding water-soluble salts can be employed, e.'g., the sodium-, potassiumor ammonium salts, and also ethanolamine salts. The latter are obtained by a partial or total neutralization of the acids.

For the pretreatment according to the invention, solutions are used whose concentrations lie between 0.1 and 10 percent by weight, and preferably 1 to 5 percent and the treatment generally is carried out at temperatures ranging from 5 to 90 C. It technologically is of advantage that this pretreatrnent can be carried out preferably at low temperatures, i.e., at a range of 5-40 C., and especially at room temperature. The treatment in the bath can be carried out for a period of 1 to 15, preferably 3 to 5 minutes.

Passivation of the metal surfaces is effected in a conventional manner. Thus, passivation baths of alkaline solutions are employed which contain alkalihydroxides, soda, phosphates, boraX and nitrites, singly or in mixture. Passivation generally is carried out at 50 to 100 0, preferably at to C. The concentration of the passivating baths is 0.3 to 3 percent by weight,- preferably 0.5 to 1.5 percent.

A special embodiment of the process according to the invention consists in effecting the etching not with hydrochloric, sulfuric or phosphoric acid, if desired, in the presence of inhibitors, as is customary, but instead with the acids of the Formulae 1 or 2 named above. These acids are applied in concentrations of 2 to 20 percent by weight at 40 to 90 C. and a period of 3 to 15 minutes. When these acids are used, etching and the pretreatment prior to enameling can be carried out in a single step.

In some instances, it is opportune to carry out an intermediate treatment after etching with solutions of metal salts whose metal ions are more noble than the metal sheet to be enameled. If iron and steel are to be enameled, solutions are applicable containing nickel, cobaltor copper salts. The concentration of the solu tions range from 0.5 to 3 percent by weight. This intermediate treatment generally is carried out at tempera tures ranging from 5 to 90 C. preferably 50 to 70 C.

3 It The treatment in the intermediary bath can be carried out for a period of 3 to 15 minutes.

When metals are treated in the manner described, pure metal surfaces, silverwhite, are obtained which do not bath temperature of 60 C. by immersion therein for minutes. The sheets then were treated for 5 minutes in a 2 percent by weight aqueous solution of 50 parts by weight gluconic acid and 50 parts aminoethane-l,l-didiscolor to a brown oxidic color upon ensuing passivation. 5 phosphonic acid at room temperature, and transferred This effect is the more surprising and unexpected as it is from there directly, without intermediary rinse, into a pasnot obtained with other complex formers, such as glycosivating bath of the composition as given in Example 1. col, lactic acid, or ethylenediaminetetraacetic acid. Equally good results were obtained by spraying in lieu The process according to the invention lends itself to of dipping.

immersion or spray technique. Example 4 fi 33 3 2 f g if f gfifi g: Well degreased iron sheets were etched in a 10 percent 3 g g"; f :3 i In 1 w S of illus HCl solution containing 6 percent ferrochloride, rinsed i s; not of eitation g s; i g g Chan 8 with water, and then immersed for approximately 5 minma made fi detail with ut awn fro a; utes in a 2 percent by weight aqueous solution of 50 s iryit 3 g f "i r weight parts aminopropane-l,l-diphosphonic acid and 50 cfaimed pe e 1 10 as B em I weight parts hydroxyethane-1,1-diphosphonic acid at ap- Exam 1e 1 proximately C. Silverwhite sheets thus were obtamed which then were passivated without intermediary Well degreased steel sheets were etched in a 10 percent 2 rinsing at 80-90 C. in a solution as named in Example by weight sulfuric acid solution containing 50 g./l. iron, 0 1. The silverwhite sheets had no more iron compounds at a bath temperature of 60 C., 'by immersion for 5 minon their surfaces and exhibited no flaws when enameled. utes, and then rinsed in water. Thereafter, the sheets N i i ti f i h d id could be observed were immersed for 4 minutes in a 2 percent by weight aqued i passivation. ous solution of equal Weight parts gluconic acid and hy- 25 Example 5 droxyethane-1,1-diph0sphonic acid at room temperature.

The ensuing passivation was carried out in a conventional Dfigreaseld Steel Sheets were etqhed y f f for bath consisting of a 1 percent by weight solution of equal 'f a 10 P y Welght f d 1 weight parts soda, phosphate, borax, and sodium nitrite, non Contalnlng 50 Iron at rinsed in Water, at a temperature of 80 C. No precipitation of iron 00- n h n m r i a 2 percent y weight i n f curred thereby, and silverwhite sheets resulted. These equal weight parts hydroxyethane-l,l-diphosphonic acid, exhibited no flaws after enameling. hydroxyhexane-1,1-diphosphonic acid and gluconic acid Equally good results are obtained when the steps at room temperature. Silverwhite surfaces were obtained named Were Carried out y P y 111 11611 of pp which do not change upon passivation as described in Example 2 1lirlivalmple l, and the enameled sheets showed no faults or Q Shae-ts deglieasedi etched i a 15 Percent Hcl Equally good results were obtained when the pretreatolutloni nnsed Wlth Water; T ereafter they, were ment according to the invention was carried out by sprayimmersed in a 2 percent by weight aqueous solution of inc instead of dipping equal parts hydroxyethane-l,l-diphosphonic acid, glucob saccharic acid and gluconic acid for 5 minutes at 25 C. Example 6 and g an mtfilmedlary Ilnse, passlyate'd at Degreased steel sheets were etched with 10 percent by 80-90 m a 5 Percent aqueous 5011mm} of equal weight aqueous sulfuric acid containing g./l. iron, weight parts soda, phosphate andborax. During (tihe P131 rinsed in water and immersed for 5 minutes in a 2 perslveitlon tll li i ih i t a td gfe siligiiziliite and 5 Cent by weight aqueous hydroxyethane-l,l-diphosphonic sur aces 0 e s ee acid solution at room temperature. The sheets then were no flaws occurred upon enamelmg.

passivated in a 0.5 percent solution as described 1n Example 3 Example 1, at -90 C. The surfaces of the silverwhite W n degreased steel h t were t h d i h a 10 sheets thus obtained contained no iron compounds and percent sulfuric acid solution containing 50 g./ 1. iron at a 50 lent themselves to flawless enameling.

TABLE 1 No. Percent Pretreatment Solution (aqueous) pH Process A Process B 2 I-Iydroxyethane-l,l-diphosphonic acid-.. 1 silverwhite, clean sheet silverwhite, clean sheet surfaces after passivation. surfaces after passivation. 2 Hydroxybutane-l,l-diphosphonic acid 0, 2 Gluconic acid Do. 1 Gluconic acid .T D0 1 Hydroxyethane-l,l-diphosphonic 801d".-. 2 Hydroxybutane-l,l-diphosphonic acid Do. 1 Hydroxybutane-l,l-diphosphonic acid D0 1 Gluconic acid 2 Aminopropane-l,l-diphosphonic acid Do. 2 Aminobenzyldiphosphonic acid (pH ad- Do.

justed with ammonia). 2 1,6-diaminohexane-1,l,6,6-tetraphosphonic D0.

acid (pH adjusted w. NH3). 2 Hydroxyhexane-l,l-diphosphonic acid..." Do. 2 Saccharic acid Do. 2 Potassium saccharate (calc. as free acid; Do.

pH adjusted w. K2603). 2 Sodium gluconate D0. 2 Sodium salt of hydroxyethanel,l-diphos- Do.

phonic acid (calc. as free acid). 2 Ethanolamine salt of hydroxyethane-1,1-- Do.

diphosphonic acid (calc. as free acid). 2 Lactic acid 2 Oxidic discoloration after Oxidic discoloration after passivation. passivationv 1 Ethylenediaminetetraacetic acid 4 do Do. 2 Monosodiumnitrilotriacetic acid 12 d0 Do.

Example 7 In the accompanying Table 1, results are listed as obtained with different pretreated solutions, as named, at otherwise like procedures and using like metal sheets.

In carrying out the comparative tests, cold rolled iron sheets first were treated at 80 C. with an alkaline cleanser, rinsed with water, and then etched at 50 C. As etching solutions, a percent by weight aqueous HCl solution (process A) or a 10 percent by weight aqueous sulfuric acid solution (process B) were employed. In process A, the solution also contained 4 percent by weight FeCl '4H O and 1 g./l. dibenzylsulfoxide as inhibitor.

In process B, 10 percent by weight FeSO -7H and 1 g./l. dibenzylsulfoxide were added to the etching solution. After etching by either method, the sheets were rinsed twice, whereby the pH in the first rinsing step was 1, in the second step 7.

In the tests 12-14, only one rinse was carried out, at a pH of 1. The sheets then were treated with the pretreatment solutions whose composition is named in the table. The ensuing passivation was carried out at 80 C. with a 0.3 percent aqueous soda solution, and finally the sheets were dried at 130 C.

The treatment time in each bath was 5 minutes, the intervening dwelling times in the air 1 minute.

Percentages listed are percent by weight.

6 cent by weight aqueous solution of equal parts soda, trisodiumphosphate and borax, at 80 0, whereby no precipitation of basic metal salts occurred. The sheets then were dried and showed a weakly bluish tempering color.

Example 11 Well degreased iron sheets were rinsed with water and then etched for 5 minutes in a 10 percent by weight aqueous aminoethane-1,1-diph0sphonic acid solution at 60 C. Upon ensuing passivation, without intermediary rinsing, in a solution as described in Example 10, no precipitation of basic metal salts occurred. The sheets, after drying, showed a weakly bluish tempering color.

We claim as our invention:

1. A process for the pretreatment of iron and steel sheet, after acid etching and prior to passivation preparatory to vitreous enameling, for the purpose of total removal of iron salts formed, which comprises exposing said sheet for 1 to 15 minutes and at temperatures ranging from 5 to 90 C. to a 0.1 to 10 percent by weight aqueous solution of a compound selected from the group consisting of polyhydrocarboxylic acids having the formulae CH OH (CHOH) 3 COOH,

CH OH (CHOH 4 COOH TABLE 2 No. Wt. Pretreatment Solution (aqueous) pH Series A Series B percent 1 2 Hydroxyethane-1,1-diphosphonic acid 8 Silverwhite surface, no pre- Matte, even copper deposit;

2 Calcined soda cipitate in test S0111. nor in no precipitate in test soln.

passivating soln. nor in passivating soln.

2 2 Gluconic acid 10 do D o 2 Calcined soda 3 2 Hydroxyethane-l-diphosphonic acid 1-2 do Do.

Example 8 Well degreased steel sheets were etched for 5 minutes in a 10 percent by weight aqueous sulfuric acid solution containing 50 g./1. iron and 1 g./l. dibenzylsulfoxide as inhibitor, at a bath temperature of 60 C., then rinsed with water. Thereafter, the sheets were treated for 10 minutes at 70 C. with a solution (series A) of 12 g./l. NiSO -H O in water, or (series B) of 15 g./l. CuSO -5H O in water. Both solutions had been adjusted to a pH of 3-3.5 with sulfuric acid, The sheets thus clad with a nickel or copper coating, respectively, then were treated for 5 minutes at room temperature by immersion in a pretreatment solution as listed in Table 2, and thereafter passivated without undergoing an intermediary rinse. Passivation was carried out at 85 C. for 5 minutes in a 0.5 percent by weight aqueous solution of equal parts soda, phosphate and bonax. The sheets then were dried at 120 C.

Table 2 shows that in all instances when the pretreatment solutions as named were applied, metal surfaces were obtained which facilitated flawless vitreous enameling. This was not the case when the pretreatment according to the invention was omitted.

Example 9 Well degreased iron sheets were rinsed with water and etched in a 7.5 percent by weight aqueous solution of hydroxyethane-l,l-diphosphonic acid at 60 C. for 5 minutes. The sheets then were passivated as described in the preceding example. They could be enameled without exhibiting any flaws.

Example 10 Well degreased iron sheets were rinsed with water and etched for 5 minutes in a 5 percent by Weight aqueous solution of 1,3-dihydroxypropane-1,1,3,3-tetraphosphonic acid at 60 C. Passiv-ation followed without intermediary rinse, in a conventional bath consisting of a 1 per- Cir COOH(CHOH) -COOH, COOH(CHOH) COOH,. gluconic acid, saccharic acid, glucosaccharic acid, acids havwherein R is selected from the group consisting of a hydrocarbon radical having 1 to 8 carbon atoms and a phenyl radical; n is a Whole number from 1 to 8; and X is selected from the group consisting of -OH and -NH water soluble salts of all those acids, and mixtures thereof.

2. The process as defined in claim 1, wherein said sheet is immersed in said solution.

3. The process as defined in claim 1, wherein said sheet is sprayed with said solution.

4. The process as defined in claim 1, wherein said pretreatment is preceded, after etching, by cladding effected by exposure to a 0.5 to 3 percent aqueous solution a compound selected from the group consisting of salts of nickel, cobalt and copper, at a pH of 3 to 3.5 at temperatures ranging from 5 to C. for 3 to 15 minutes.

5. The process as defined in claim 1, wherein said acid etching is carried out for 3 to 15 minutes at 40 to 90 C.

using a 2 to 20 percent aqueous solution of an acid selected from the group consisting of 0 R o HOJ MLOH and 55 wherein R is selected from the group consisting of a hydrocarbon radical having 1 to 8 carbon atoms and a phenyl radical; n is a Whole number from 1 to 8', X is selected from the group consisting of -OH and -NH 6. The process as defined in claim 5, wherein etching and said pretreatment are combined in a single step.

References Cited by the Examiner UNITED STATES PATENTS 5/1943 Percival 156-18 X 3/1961 Davis et *al 1-17-50 

1. A PROCESS FOR THE PRETREATMENT OF IRON AND STEEL SHEET, AFTER ACID ETCHING AND PRIOR TO PASSIVATION PREPARATORY TO VITREOUS ENAMELING, FOR THE PURPOSE OF TOTAL REMOVAL OF IRON SALTS FORMED, WHICH COMPRISES EXPOSING SAID SHEET FOR 1 TO 15 MINUTES AND AT TEMPERATURES RANGING FROM 5 TO 90*C. TO A 0.1 TO 10 PERCENT BY WEIGHT AQUEOUS SOLUTION OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF POLYHYDROCARBOXYLIC ACIDS HAVING THE FORMULAE CH2OH(CHOH)3.COOH, 